Process of separating mixtures of particles



A ril 12, 1960 P, MAROT 2,932,395

PROCESS OF SEPARATING MIXTURES OF PARTICLES Filed NOV. 16, 1954 I PROCESS OF SEPARATING MIXTURES OF PARTICLES Paul Marot, Uccle, Belgium, assignor to Stamicarhon N.V., Heerlen, Netherlands Application November 16,1954, Serial No. 469,271

Claims priority, application Netherlands November 21, 1953 8 Claims. (Cl. 209-1725) This invention relates to particle separation and more particularly to a process of separating mixtures of particles diflfering in size and specific gravitywherein the mixture is separated into a fine and coarse fraction by subjecting it to a wet-screening treatment, and thereafter the two fractions are separated according to specific gravity with the help of a separating suspension. The

nited States Patent ice ' from the magnetic separator 12, are supplied to a mixer invention is especially applicable in washing non-deslimed coal or crude ore.

In the known processes, the wet-screening treatment is carried out with water. Asa result, the separating suspension is continuously diluted with water, so that it is necessary to keep the specific gravity of the separating suspension at the desired value. Furthermore, in the application of the known process, the fine fraction obtained by the wet-screening treatment must be very finely.

screened in order to remove at least part of the water contained therein. However, screening according to a very fine mesh is often costly, especially if the mixture to be separated contains a large amount of very fine particles. In the latter case, the Water consumption will also be very high.

It is therefore an object of the present invention to provide a novel process for separating mixtures of particles difiering in size and specific gravity wherein the initial wet-screening treatment is carried out with the use of a separating suspension capable of subsequent use in the separation of the resulting fine fraction so as to eliminate the necessity .of screening the latter.

Another object of the present invention is the pro,- vision of an improved process for separating mixtures of particles differing in size and'specific gravity wherein the separating suspension is recycled for continuous use in substantially all of the steps of the process.

A further object of the present invention is the provision of a process of the type described wherein a separating suspension comprising fine magnetic particles is used and recycled'so that a minimum of the magnetic particles will be lost and a minimum fresh supply of the suspension is required for each cycle.

These and other objects of the present invention will become more apparent during the course of the follow ing detailed description and appended claims.

The invention may best be understood with reference to the accompanying drawing wherein afiow diagram is shown illustrating one example of the manner in which. the present invention may be carried out.

The fiowsheet represents a coal washery in which nondeslimed raw fine coal (-10 mm.) is washed with a magnetite suspension or fine magnetic particles suspended in a liquid. At a, the coal ,is supplied to a desliming screen 1 over which sprayers 2 have been arranged. The raw fine coal is sprayed oif with magnetic suspension;-the fine fraction is collected in a reservoir 3, the coarse fraction is passed to a mixing tank 5 along a pipeline 4. Leaving reservoir 3, the fine fraction suspended in separating suspension is fed through pipeline 15, the former flowing along a pipeline 13 and the latter along the pipelines 14 and 13, wherein a suspension of the desired specific gravity is prepared by supplying water through a pipeline 16 and valve 17.

From the mixer 15 sufiicient suspension is fed to the sprayers 2 by means of a How dividing box 18, to thereby keep the level of the suspension in the reservoir 3 approximately constant. This amount is controlled by means of a valve 19 in a pipeline 20, through which the remainder is carried away to a thickener 21. The fraction of non-magnetics from the magnetic separator 10 (the fine coal fraction) passes through pipeline 22 to a second magnetic separator23 where some of the remaining magneticis separated off and subsequently supplied to the thickener 21 along a pipeline 24. 'The fraction of non-magnetics from the magnetic separator 23 (the fine coal fraction) passes through a pipeline 25 by a pump 26 into a hydrocyclone 27 wherein it is thickened. Water, which still contains a very small amount of magnetite together with the finest coal particles (e.g. microns), is carried oil through pipe 28, and the thickened coal fraction passes along pipeline 29 to a dewatering centrifuge 30 from which a fine coal fraction is obtained at b. The water from centrifuge 30 containing very fine particles is fed to the thickener 21 along pipeline 31. t

In the event that the pump 26 should receive insufficient liquid, part of the overflow fraction from the hydrocyclone 27 may be returned to the pipeline 25 along a pipe 32 and valve 33. Moreover part of this fraction may be fed to the thickener 21 along a pipeline 34 and valve 35. ,7

The fraction of non-magnetics from the magnetic separator 12 (the fine shales fraction) passes through a pipeline 36 to a second magnetic separator 37 where some of the remaining magnetite is separated off and passed through a pipeline 38 into pipeline 24 which feeds into the thickener 21.

The fraction of non-magnetics from the magnetic separator 37 (the fine shales fraction) passes through a pipeline 39 and a pump 49 to a hydrocyclone 41 where this fraction is thickened. Water, which still contains a very small amount of magnetite, together with the finest shale particles (e.g. 150 microns), is carried off through a pipeline 42, and the thickened shale fraction passes along pipeline 43 to a dewatering centrifuge 44 from which a shale slimes fraction is obtained at c, the water therefrom containing very fine particles being fed -to the thickener 21 through a pipeline 45. Part of the overflow fraction from the hydrocyclone 41 may be returned to the pipeline 39 through a pipeline 46 and a valve 47. Another part of this fraction may be fed to the thickener 21 along pipeline 48 and valve 49.

In the mixing tank 5 the coarse fraction is mixed with separating suspension supplied through a pipeline 50. The mixture comprising the coarse fraction and separating suspension, passes through a pipeline 51 to a hydrocyclone 52 whose axis is set at a small angle to the horizontal.

Under these conditions a small feed pressure (about 4.5 m. of suspension column) suflices for separating the infed mixture in the hydrocyclone. Hence, by arranging I the mixing tank 5 at a convenient height over the hydrohydrocyclone 52 without making use of a pump. The

overflow fraction from the hydrocyclone 52, the coal fraction, is first passed across a draining screen 53 and subsequently across a washing screen 54. Over the washing screen 54, Sprayers S and 56 are arranged, the former receiving clarified liquid through the pipeline 28 and the latter being fed with water. The washed coal is carried off at d and the diluted suspension collected under the screen 54 is fed to the magnetic separator through a pipeline 57.

The apex fraction from the hydrocyclone 52, the shale fraction, is first passed across a draining screen 58 and subsequently across a washing screen 59 having sprayers 60, receiving clarified liquid through the pipeline 42, and sprayers 61 which are fed with water, arranged thereover. The shale is carried off at e and the dilutedsuspension which is collected under the screen 59 is supplied to the magnetic separator 12 through a pipeline 62. The undiluted separating suspension collected under the screens 53 and 58 is fed to a suspension reservoir 64 along a pipeline 63. The reservoir also receives the thickened suspension from the thickener 21 through a pipeline 65, pump 66 and valve 67. From the suspension reservoir 64, the suspension is fed to the mixing tank 5 through a pump 68 and a pipeline 50. Disposed in the mixing tank 5 is an overflow compartment 69 having an opening provided in the bottom thereof. Part of the separating suspension enters the mixing tank 5 through this compartment 69. The total amount of suspension supplied through the pipeline 50 is larger than can be dealt with by the hydrocyclone 52, so that part of the suspension fed into the compartment 69 will flow into the pipeline 70. In this manner, the liquid level in the mixing tank 5 can be kept constant without particles from the mixture getting into the overflow. The overflow from the compartment 69 returns partly to the suspension reservoir 64 through a flow dividing box 71 and a valve 72, and partly to the mixer through the flow dividing box 71 and a pipeline 73. Through a pipeline 74 having a magnetizing coil 75, fresh magnetite is supplied to the thickener 21. The overflow from the thickener 21 is discharged at f.

Usually the overflow fraction 1 is subjected to a further treatment. This is desirable because the overflow consists of a strongly diluted suspension of very fine coal and shale particles which may also contain a small amount of fine magnetite. The fraction 7 may be fed, for example, to a small froth flotation plant in which the fine coal is separated oif. Subsequently, the froth is supplied for example to a (Imperial) filter. The resulting filtrate may then be returned to the thickener 21. The tailings containing water from the froth flotation plant may be flocculated in a thickener, and the tailings then dried in a filter press. The overflow water from the flocculating thickener may be re-used for feeding the sprayers 56 and 61 and/or for grinding fresh magnetite and/ or be returned into the pipeline 16. The water losses may then be made up in the overflow tank of the flocculating thickener. The treatment of the fraction f has not been indicated in the drawing, as this would tend to make the drawing unduly complicated. Moreover, several reservoirs and discharge pipes, which naturally are indispensable in practice, have likewise been omitted from the drawing. For example, the thickener 21, will also serve as a collecting reservoir for the spillage water. Moreover, part of the diluted suspension from the spraying screens 54 and 59 may be fed to the thickener.

The system is controlled as follows. The separating suspension which is supplied to the mixer 15 through pipelines 13 and 73 should have a specific gravity not lower than that necessary for eifecting the separation in the hydrocyclone 8. The exact specific gravity may be obtained by adjusting the valve 17. The level in the reservoir 3 'is kept constant by means of the valve 19.

The thickened suspension from the thickener 21 should have a specific gravity higher than is needed for effecting the separation in the hydrocyclone 52. The specific gravity in the suspension reservoir 64 is then controlled by means of the valve 67. The level of the liquid in the suspension reservoir 64 may be regulated by means of the valve 72. The operation of the hydrocyclones 27 and 41 depends on the size of the apex aperture.

Preferably the entire control process will be carried out automatically. The thickener 21 should be so dimensioned that the magnetite sinks and the fine coal and shale particles get into the overflow. These dimensions naturally depend on the capacity of the thickener. It is important that the material fed into the magnetic separator 10 should have an appropriate consistency, that is, the content of solids in the feed should be neither too low (as this would result in an insufiicient capacity of the magnetic separators) nor too high (unfavorable effect upon the sharpness of separation). Therefore, the amount of suspension supplied through the pipeline 9 should be in a certain proportion to the amount of diluted suspension supplied through the pipeline 57. The same holds for the amounts supplied through the pipelines 11 and 62. These amounts are primarily dependent on the amounts treated in the hydrocyclones 8 and 52 respectively, that is on the grain size distribution of the raw coal and on the mesh of the screen 1.

It can thus be seen that there has been provided a process for separating mixtures of particles differing in size and specific gravity wherein the wet screening treatment is carried out by the use of a separating suspension which will not be diluted in the resulting fine fraction thereby eliminating the heretofore necessary step of screening the resulting fine fraction.

Preferably, the process according to the invention is characterized by the following features:

(a) The separating suspension consists of fine, magnetic particles suspended in liquid;

(b) The fine particles fraction is separated according to specific gravity in a hydrocyclone;

'(c) The resulting fractions are each treated in a magnetic separator;

'(d) The fractions of magnetic particles thus obtained are, at least substantially, utilized in the wet-screening treatment;

(e) The particles of the mixture if desired with the exception of the very finest are separated from the fractions of non-magnetic particles;

(f) The coarse particle fraction obtained from the wet-screening operation is treated in an apparatus adapted to effect aseparation by means of a separating suspension, said apparatus being hereinafter termed suspension separator, and the separated fractions are screened, whereafter the undiluted suspension thus obtained-or at least the greater part thereofis returned to the suspension separator;

:(g) The coarse particles are subsequently sprayed ofi on washing screens; at first with the liquid drawn from the fractions of non-magnetic particles and afterwards with water;

(h) At least part of the diluted suspension from the washing screens is supplied to the magnetic separators: the diluted suspension from the washing screen for the specifically light particles to the magnetic separator for specifically light particles and the diluted suspension from the washing screen for specifically heavy particles to the magnetic separator for specifically heavy particles; and

(i) Liquid containing a minimum of magnetic particles is drained from the aforementioned circuits.

This system has several advantages. For example the amount of separating suspension to be used in the separating circuit for the fine particles may be small, because in the hydrocyclone where the specific gravity separation takes place it is possible to operate at a high concentration of the particles to be separated, even if the quantity of very fine particles occurring in this fine particle fraction should be large on the average. Furthermore, where due to disintegration and errors made in the'wet-sc'reening treatment, fine particles have gotten into the circuit for the coarse particles, they are subsequently introduced into the circuit for the fine particles; the specifically light particles being added to the fraction of fine specifically light particles, the specifically heavy particles joining the fraction of fine specifically heavy particles.

In this manner these particles are kept separated from one another and are carried oif together with the other specifically light and specifically heavy particles, respectively. Moreover, due to the addition of diluted suspension, the fractions obtained from the hydrocyclone are more adaptable to treatment in the magnetic separators. In addition, all of the separating suspension in the circuit for the fine particles is subjected to a magnetic purification in the course of each cycle, so that no accumulation Y of impurities can occur.

It is preferred to separate the coarse particles in a hydrocyclone which is positioned so as to have its axis substantially horizontal, and for this separation to use a suspension having a slightly higher specific gravity than the suspension employed for separating the fine particles. This is advantageous in that the separation of the coarse particles requires but little energy. The specific gravity of separation for the coarse particles is substantially equal to the specific gravity of separation for the fine particles, since during the washing process in a hydrocyclone, the specific gravity of separation is dependent on the particle size. Consequently, if use is made of the same separating suspension, the specific gravity of separation for the fine particles will be higher.

To reduce the magnetite losses, it is important that the fraction of non-magnetics from the first magnetic separator be treated in a second magnetic separator, and that the fraction of magnetics obtained from said second magnetic separator be supplied to a thickener and the thickened fraction obtained therefrom be reused in the separation process either for separating the coarse particles or for separating the fine particles, or for separating both the coarse and the fine particles.

The magnetic losses may be reduced still further by thickening the fractions of non-magnetics from the magnetic separators in hydrocyclones, and by subjecting the fractions thus obtained to a further de-watering process, and by subsequently supplying part ofthe liquid obtained in these treatments, which liquid may contain very fine particles, to the aforementioned thickener, where the magnetics are caused to sink and the very fine nonmagnetics are caused to flow across the overflow edge.

'It is necessary of course to control the various operations in such a way that the amount of liquid and the amount of magnetics in each circuit will remain substantially constant. One of the consequences of this condition is that the liquid with which the coarse specifically heavy particles are first sprayed oii should be obtained from the fraction of specifically heavy non-magnetics, and that the liquid with which the coarse specifically light particles are first sprayed off should be obtained from the fraction of specifically light non-magnetics.

When fine coal smaller than mm. is being treated the screen 1 may in many cases make a separation according to a particle size of about 2 mm.

The following data concern a washery which is adapted for washing 140 tons of fine coal with a particle size of l0 mm. per hour:

Screen 1: 2 Allis Chalmers Low Head screens, dimensions 6' x 16, size of mesh 1.5 X 20 mm.

Pump 6: capacity 255 cub. m./h.; feed pressure: 14 m. of suspension column near the hydrocyclone 8.

Magnetic separators 10, 12', 23 and 37: numbering 7, 2, 4 and 1 respectively, all being 4' wide.

Centrifuges 30 and 44: Escher Wyss.-

Pump 68: capacity 255 cub. m./h. Thickener 21: diameter 5 m. Pump 66: capacity 15 cub. m./h.

Dimensions of hydrocyclones Hydrocyclones (number) 8 52 27 41 Numbers 2 2 5 1 Diameters cylindrical part (mm.) 350 500 350 350 Height of cylindrical part (mm.)

Apcx angle, degrees 20 20 20 20 Diameter feed opening (mm.) 70 100 70 70 Diameter vortex finder (mm. 150 216 70 150 Length vortex tinder (mm.) 150 200 150 150 Diameter apex opening (mm.) 150 Angle between aids and vertical, degrees 0 75 0 0 Feed pressure (at overpressure) 1. 0 0. 5 Feed pressure (suspension column in m.) 14 4. 5

1 Variable.

of the magnetite supplied through the pipeline 74 has a particle size smaller than 40 microns. The specific gravity of the separating suspension in the reservoir 3 is adjusted at about 1.55, in the reservoir 64 at about 1.75. The fraction of magnetics obtained from the magnetic separators 10 and 12 has a specific gravity of about 1.8, the specific gravity of the thickened suspension obtained from the thickener 21 being about 2.0.

The operation of the hydrocyclones 27 and 41 is so controlled that only particles smaller than 150 microns (or microns if desired) will get into the over flow fraction. z

It is to be understood that the example of the invention herewith shown and described is to be taken as the preferred embodiment of the same and various changes may be made without departing from the spirit of the invention and the scope of the appended claims.

I claim:

l. A process of separating a mixture of particles differing in particle size and specific gravity which comprises subjecting said mixture with a separating suspension including fine magnetic particles suspended in a liquid to a wet-screening treatment to obtain a fine fraction and a coarse fraction, separating said fine fraction in a hydrocyclone to obtain a fine specifically light fraction and a fine specifically heavy fraction, treating each of said fine fractions in magnetic separator means to obtain fractions of magnetic particles and fractions of non-magnetic particles, utilizing at least a part of the fractions of magnetic particles in said wet-screening treatment, separating said coarse fraction in a suspension separator to obtain a coarse specifically light fraction and a coarse specifically heavy fraction, screening each of said coarse fractions to obtain fractions of coarse particles and fractions of undiluted suspension, returning a substantial part of said fractions of undiluted suspension to said suspension separator, spraying ofi each of said fractions of coarse particles on washing screens, first with liquid obtained from the fractions of non-magnetic particles and afterwards with water, and feeding at least a part of the diluted suspension from the washing screens to said magnetic separator means, the diluted suspension from the washing screen for the specifically light fraction of coarse particles being fed to the magnetic separator means for the fine specifically light fraction and the diluted suspension from the washingscreen for the specifically heavy fraction of coarse particles being fed to the magnetic separator means for the fine specifically heavy fraction. V

' 2. A process as defined in claim 1 wherein the suspension used in separating said coarse fraction has a higher specific gravity than the suspension used in separating said fine fraction.

3. A process as defined in claim 1 wherein said magnetic separator means comprises first and second magnetic separators in series, thefractions of magnetic particles obtained from said second magnetic separators are supplied to a thickener and the thickened fraction obtained therefrom is reused in the separation process.

4. A process as defined inclaim '3 wherein said fractions of non-magnetic particles are thickened in hydrocyclones and the thickened fractions are subjected to a further dewatering treatment and the liquid thus obtained is supplied to said thickener.

5. A process as defined in claim 1 wherein the liquid with which:the specifically light fraction of coarse particles is first sprayed oif is drawn from the specifically light fraction of non-magnetic particles and the liquid with which the specifically heavy fraction of coarse particles is first sprayed ofi is drawn from the specifically heavy fraction of non-magnetic particles.

6. A process as defined in claim 1 wherein said fractions of non-magnetic particles are thickened in a hydrocyclone, the thickened fractions are subjected to a further dewatering treatment and the liquid thus obtained is supplied to a thickener.

7. A process of separating a raw mixture of non-magnetic particles differing in particle size and specific gravity which comprises the steps of subjecting the raw mixture to a wet screening treatment in which the mixture is sprayed with a separating suspension which includes finely ground magnetic particles suspended in a liquid to thereby obtain a fine suspended fraction and a coarse fraction, effecting a heavy media separation of said fine suspended fraction in which the suspension in said fine fraction constitutes a heavy medium of a specific gravity intermediate that of the non-magnetic particles in said fine fraction tothereby obtain specifically light and specifically heavy fractions separating the magnetic particles of said suspension from said specifically light and specifically heavy fractions, and recirculating the magnetic particles separated from said fine specifically light and specifically heavy fractions for use in said wet screening treatment.

8. A process of separating a raw mixture of non-magnetic particles differing in particle size and specific gravity which comprises the steps. of subjecting the raw mixture to a wet screening treatment in which the mixture is sprayed with a separating suspension which includes finely ground magnetic particles suspended in a liquid to thereby obtain a fine suspended fraction and a coarse fraction, effecting a heavy media separation of said fine suspended fraction in which the suspension in said fine fraction constitutes a heavy medium of a specific gravity intermediate that of the non-magnetic particles in said fine fraction to thereby obtain fine specifically light and specifically heavy fractions separating the magnetic particles of said suspension from said fine specifically light and specifically heavy fractions, recirculating the magnetic particles separated from said fine specifically light and specifically heavy fractions 'for use in said wet screening treatment, mixing separating suspension with said coarse fraction, efiecting a heavy media separation of said coarse fraction in which the suspension in said coarse fraction constitutes a heavy medium of a specific gravity intermediate that of the non-magnetic particles in said coarse fraction to thereby obtain coarse specifically light and specifically heavy fractions, separating the suspension from said coarse specifically light and specifically heavy fractions, and recirculating the suspension separated from said coarse specifically light and specifically heavy fraction for mixture with said coarse fraction.

References Cited in the file of this patent UNITED STATES PATENTS 2,151,175 Wuensch Mar. 21, 1939 2,387,866 Walker Oct. 30, 1945 2,696,299 Van Os Dec. 7, 1954 2,701,641 Krijgsman Feb. 8, 1955 

